1. Field of the Invention
The present invention relates to a process for production of gas hydrate for producing gas hydrate that is a hydration reaction product of raw material gas and water.
2. Description of the Related Art
The gas hydrate is an icy solid crystal consisting of a water molecule and a gas molecule, and a general term for clathrate hydrate in which the gas molecule is present in a steric cage structured by the water molecules. It is said that the gas hydrate can contain natural gas in an amount as large as approximately 165 Nm3 per unit volume of 1 m3. For this reason, research and development for utilizing the gas hydrate as natural gas transport and storage means are intensively performed.
Advantages of the natural gas being hydrated include: (a) enabling storage and transport under a temperature condition easier than a storage and transport temperature (−163° C.) of liquefied natural gas (LNG) under atmospheric pressure, which has been put into practical use; and (b) enabling durability or heat insulating properties of storage and transport equipment to be significantly simplified because the gas hydrate has self-preservation property.
In general, the gas hydrate can be produced on a lower temperature and higher pressure side of a three-phase equilibrium curve of hydrate, water, and gas. When the gas hydrate is produced from water and raw material gas, it is generally produced under a temperature condition equal to or more than 0° C. and a pressure condition higher than an equilibrium condition. On the other hand, from a perspective of emphasizing economic efficiency, pressure upon storage and transport is preferably lower.
A production condition of the gas hydrate typically includes a pressure of 1 to 5 MPa and a temperature of 0 to 10° C. However, If the gas hydrate having been produced under a pressure condition higher than the equilibrium condition is depressurized to a pressure lower than the equilibrium condition, e.g. to atmospheric pressure for storage or transport, the gas hydrate is decomposed in the process of the depressurization, so that there has been proposed a technique in which the gas hydrate is cooled to below freezing point and extracted with being frozen along with adhered water (see, for example, Patent documents 1 and 2).
The extracted hydrate is superior in economic efficiency if it contains a larger amount of gas, so that it is important to reduce an amount of gas discharged as much as possible in the process of the depressurization. Also, if the gas hydrate is cooled to below the freezing point, refrigeration equipment and running cost cause an increase in burden, and therefore we have checked from experiment a most appropriate extent to which the gas hydrate is cooled upon depressurization (pressure release) of the gas hydrate. The experimental results are illustrated in FIGS. 5 and 6.
It turns out from the diagrams that a decomposition amount of gas hydrate varies depending on a type, or concentration of additive gas mixed into methane, or a cooling temperature. For example, if the additive gas is ethane, the gas hydrate is hardly decomposed at the cooling temperature of −5° C. as illustrated in FIG. 5. However, it also turns out that at the cooling temperature of −10° C., the gas hydrate is decomposed at a rate of approximately 5 to 28%, and at −25° C., the gas hydrate is decomposed at a rate of approximately 1 to 43%.
In addition, if the additive gas is propane, it turns out as illustrated in FIG. 6 that at the cooling temperature of −5° C., the gas hydrate is decomposed at a rate of approximately 3 to 35%; at −10° C., at a rate of approximately 9 to 30%; and at −25° C., at a rate of approximately 1 to 35%.
When the gas hydrate is produced with mixed gas (raw material gas) of methane including ethane and propane component, it turns out that the gas hydrate of which structure types I and II coexist is produced, and the structure type II contains mixed gas of methane and propane, or methane and ethane, of which a concentration in the structure is 20 to 30%.
A self preservation principle of gas hydrate is considered as follows:
(a) When the gas hydrate having been produced under high pressure is frozen and depressurized to be brought into a decomposition condition under atmospheric pressure, the decomposition of the gas hydrate is partially started from its surface, and gas molecules forming the gas hydrate are gasified, as well as a water film covers the gas hydrate surface.
(b) When heat is lost due to the decomposition at the gas hydrate surface, the water film on the gas hydrate surface comes to an ice film which covers the gas hydrate surface.
(c) When the ice film grows to a certain thickness or more, heat exchange between the gas hydrate inside the ice film and the outside is blocked, and therefore the inside gas hydrate is stabilized even under the decomposition condition such as atmospheric pressure.
(d) That is, because the ice film has mechanical strength sufficient to resist pressure of the decomposing gas hydrate, the gas hydrate is stabilized, and further decomposition is suppressed.
Note that, preferably, the decomposition at the gas hydrate surface rapidly progresses to form the ice film on the gas hydrate surface. On the other hand, if the decomposition at the gas hydrate surface slowly progresses, the decomposition progresses to the inside before the ice film is formed on the gas hydrate surface, and consequently the decomposition amount upon depressurization is increased
Accordingly, by depressurization at a temperature higher than the equilibrium temperature of the gas hydrate by a certain degree or more, stable ice is formed as a film on the gas hydrate surface upon the depressurization. If a shift in temperature is small, the decomposition slowly progresses, so that the ice growing on the gas hydrate surface does not form a film, and therefore the decomposition amount is increased. From the experimental results, it turns out that by setting the shift in temperature to 40 degrees or more from the equilibrium temperature upon the depressurization, the ice film is formed, and therefore the decomposition is suppressed.    Patent document 1: Japanese patent application Kokai publication No. 2001-280592    Patent document 2: Japanese patent application Kokai publication No. 2003-105362